At the cutting edge of todays security research and development, the SHA-3 contest evaluates a new successor of SHA-2 for secure hashing operations. One of the finalists is the SHA-3 candidate
. Like many other cryptographic primitives Skein utilizes arithmetic operations, for instance modular addition. In this paper we introduce a new method of performing a DPA on modular addition of arbitrary length. We will give an overview over side channel analysis of modular addition, followed by problems occurring when dealing with large operand sizes of 32 bits and more. To overcome these problems, we suggest a new method, called the
to exploit the leakage of modular additions. Real world application is being shown by applying our new approach to Skein-MAC, enabling us to forge legitimate MACs using Skein.